(1) Field of the Invention
The present invention relates to a hydraulic fluid supply system for supplying a hydraulic fluid under pressure to a hydraulic actuator. The hydraulic fluid supply system according to the present invention is particularly suitable for supplying a controlled flow of a high pressure hydraulic fluid to power cylinders and servomotors of automotive power steering systems.
(2) Description of the Related Art
Modern automobiles are often provided with power steering systems to assist a driver when turning the steering wheel. The power steering system includes various types of hydraulic actuators, including a hydraulic power cylinder and hydraulic servomotor operatively connected with the steering linkage of the steering system. In allmost all instances, the hydraulic actuator is provided with a high-pressure hydraulic fluid by means of an engine-driven hydraulic pump such as a rotary vane pump.
Normally, the hydraulic pump used as a power source for the power steering system is a fixed displacement type, so that the output of the pump varies considerably in proportion to variations in the engine speed. Accordingly, for the power steering system to develop sufficient hydraulic power during engine idling, the pump must be designed to have a displacement capacity such that it will deliver a predetermined minimum output when the pump is running at minimum speed. This results in that the pump discharges an excessive amount of fluid at elevated engine speeds.
To counteract this excessive discharge, a flow control valve is usually provided between the pump and hydraulic actuator to control the flow rate of the hydraulic fluid at predetermined values regardless of the engine speed, and hence, the discharge rate of the pump.
Another requirement in the design of the power steering system is the provision of reduced hydraulic power during high speed cruising of the vehicle, to ensure the heavy steering required to counteract any abrupt steering action by the driver. This has been achieved by reducing the flow rate of the hydraulic fluid supplied to the power steering actuators at increased engine speeds. Such a reduction in the fluid supply rate at high engine speeds is known in the art as "drooping".
Another important requirement is to provide a power steering system operable at a minimum power consumption.
Japanese Unexamined Patent Publication (Kokai) No. 57-116910, published July 21, 1982, and entitled "Apparatus for supplying a high pressure hydraulic fluid", discloses a hydraulic fluid supply system provided with a "drooping" mechanism and capable of being operated at a reduced power during drooping. This publication discloses a fluid supply system comprising two independent pumps. The first pump is connected to a main passage having a fixed orifice and communicated with the power steering system, and a flow control valve having a variable orifice is positioned across the main passage. The second pump is connected to a subpassage which is selectively connected is response to the position of the flow control valve to a return passage or to the main passage. When the rotational speed of both pumps is increased, which, in turn, increases their output, the spool valve is moved to connect the subpassage with the return passage, thereby permitting the pressure fluid from the second pump to return to the reservoir by bypassing the power steering system. Thus, the power steering system is "drooped" when the pumps is running at a high speed, and the power consumption is correspondingly reduced.
The disadvantage of the system disclosed in the foregoing publication is that the rotational speed of the power pump and engine is not necessarily proportional to the vehicle speed. For example, the engine speed, and hence, the pump speed, varies considerably according to the position selected in the transmission system. Thus, under certain circumstances, the power pump and engine are operated at a low speed although the vehicle is running at a high speed. In this situation, the disclosed fluid supply system is not capable of achieving "drooping" and thereby reducing the power consumption.